SANTA CRUZ — At the request of city police, the FBI on Monday joined an investigation into the weekend riot downtown, which authorities say was a “methodical and coordinated” attack by anarchists.

Police sought the FBI’s help because investigators believe the Saturday night May Day street party that drew 250 people, an unsanctioned event advertised by fliers placed around town and at UC Santa Cruz, was designed to provide “political and physical” cover for an unnamed anarchist group.

Masked demonstrators grabbed large rocks from Pacific Avenue tree beds and threw them through business windows. They also scrawled anarchist graffiti on buildings and jumped on top of a police car, pelting it with rocks and covering it with paint.

Police also revealed Monday that they received two 911 calls about shooting incidents that night, which were later determined to be unfounded. Investigators believe the calls were meant to divert officers away from downtown, where 18 businesses, half of them small locally owned enterprises, eventually sustained damage estimated at $100,000.

“It was coordinated to split up our resources,” said Police Chief Howard Skerry, adding that his department is already understaffed because of the city budget crisis. “We need more people on the street.”

Council members will discuss the matter today with City Manager Dick Wilson, who is charged with closing a $3 million to $4 million budget deficit by July. Wilson already has approved overtime expenses to investigate the case, as well as an April 23 gang-related homicide on the Westside.

“We have a multimillion-dollar budget deficit next year,” Wilson said. “We don’t have any money to do anything with.”

Joseph Schadler, a spokesman for the FBI office in San Francisco, confirmed the agency joined the probe to determine whether any federal laws were broken. Authorities declined to say whether there are any suspected connections between the riot and a series of animal rights-related demonstrations targeting UC Santa Cruz scientists in 2008, which included firebombings that drew FBI agents into the case.

Police said Monday that authorities made a second arrest in connection with Saturday’s riot, but the man’s identity was not confirmed until Monday. Thomas Williams, who police described as a 41-year-old transient, was caught near the Town Clock, covered in paint similar to that used in the vandalism. He was arrested on charges of obstructing police and public intoxication.

Saturday police arrested 24-year-old Jimi Haynes, a Fresno County transient wanted on a parole violation for burglary. He was charged with felony vandalism.

Police said they are not sure who organized the event, which was advertised as a “massive street party” to celebrate International Workers Day. Some of the fliers promise the opportunity to “kick it, eat, drink, dance, take over.” Another flier described an opportunity for a “truly sick night of mayhem.”

After police learned about the fliers several weeks ago, they followed department procedure and asked the city’s special event coordinator, Kathy Agnone, to contact the group to find out more about their plans. The police department changed its policies on investigating First Amendment activities several years ago after a controversial undercover probe of the anarchist-themed Do It Yourself parade.

Police are allowed to investigate groups or planned events they suspect involve criminal activity, but police leaders say there was no reason to suspect the May Day event was a cover for vandals. The event fliers, which were posted around downtown and circulated online, contained anarchist language similar to fliers circulated about the DIY parade, which has never turned violent.

“This is the first time we have had the experience of people who call themselves anarchists joining another function,” Wilson said.

Wilson said the city will investigate its procedures for investigating non-permitted functions, a role that used to be performed by police until the undercover operation involving DIY in 2005. Agnone, the special events coordinator, said she posted a comment on the May Day event’s website encouraging organizers to seek a city permit, but she received no response.

Agnone said some of the language in the fliers “was a concern for sure,” but she did not remember warning other officials. She said she sometimes contacts police or parks officials about the potential size of crowds or effect on traffic, but ultimately, “What I look at is time, place and manner, and often content is not scrutinized.”

At UC Santa Cruz, the theme “Dance Party” has become synonymous with anarchist activity. Last year’s occupations of Kerr Hall and the Graduate Student Commons were originally advertised as dance parties. In both cases, masked demonstrators stormed the buildings and held office spaces for several days. The Kerr Hall occupation ended with police in riot gear forcing protesters out of the building.

Campus spokesman Jim Burns said he didn’t know whether any student affairs officials noticed the fliers and were concerned.

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